A common battery includes: an electrode assembly comprising a positive electrode, a negative electrode, and a separator disposed between the two electrodes; a battery case housing the electrode assembly; a seal member sealing the battery case; a negative electrode lead connecting the negative electrode with the bottom of the battery case; and a positive electrode lead connecting the positive electrode with the seal member. For example, a nickel lead is used as the negative electrode lead.
When such a battery short-circuits externally, the nickel negative electrode lead, which has the highest resistance, is prone to generate heat. Of the negative electrode lead, the bent portion (the border between the part facing the inner side face of the battery case and the part facing the inner bottom face of the battery case and the adjacent portion), in particular, has a higher resistance than the other portions and generates a large amount of heat, since the bent portion is stretched due to plastic deformation. Also, since this portion is not in contact with the battery case, the heat generated by the negative electrode lead is unlikely to be released to outside through the battery case, and the battery temperature rises significantly.
In order to suppress the generation of heat by external short circuits, for example, Japanese Laid-Open Patent Publication No. Hei 10-214614 proposes a battery which is different in structure from the above-described battery. The proposed battery includes an electrode assembly, a battery case housing the electrode assembly, positive and negative electrode terminals provided on the battery case, a negative electrode lead connecting the negative electrode of the electrode assembly with the negative electrode terminal, and a positive electrode lead connecting the positive electrode with the positive electrode terminal. In this battery, a part of the negative electrode lead is provided with a narrow portion, so that in the event of an external short circuit, the narrow portion of the negative electrode lead is melted and broken to interrupt the current.
However, when such proposal is applied to, for example, a cylindrical battery, even if the narrow portion of the negative electrode lead is broken due to melting, the broken portions may adhere to each other again, thereby allowing the current to flow again to produce heat. In this case, since the narrow portion is not in contact with the battery case, the heat generated by the negative electrode lead is unlikely to be released from the battery, and the battery temperature rises significantly.